Its without error...! What I mean is, when you programm a software then you want to test it. I know there are a lot of kind of testing. For example unit testing....! I think I will read something about unit testing...!

Its without error...! What I mean is, when you programm a software then you want to test it. I know there are a lot of kind of testing. For example unit testing....! I think I will read something about unit testing...!

There are indeed a lot of kinds of testing. Unit testing being one of them at the code level. Others include unit integration testing, funtional testing and user acceptance. I understand your question: your code compiles, your program runs and you get some output, yet : is the output correct? Does each function and class does what it is meant to do?
This means that you can test functions and classes separately.

I can advise you to look at TDD (Test Driven Development). This paradigm says that you design tests first before you start coding. You keep running your testsuite until all tests are passed, if not: keep changing the code.
Also: look at the terms "stub" and "driver"